Amplifying apparatus providing two output states



April 25, 1967 HULL 3,316,423

AMPLIFYING APPARATUS PROVIDING TWO OUTPUT STATES Filed July 2, 1964 WITNESSES INVENTOR %,%?M. Robert E. Hull United States Patent 3,316,423 AMPLIFYING APPARATUS PROVIDING TWO OUTPUT STATES Robert E. Hull, Amherst, N.Y., assignor to Westinghouse Electric Corporation, Pittsburgh, Pin, a corporation of Pennsylvania Filed July 2, 1964, Ser. No. 379,983 8 Claims. (Cl. 30788.5)

This invention relates to amplifying apparatus selectively operable to either of two output states, and more particularly to a voltage sensitive relay circuit employing amplifying valves for example, transistors.

It is an object of this invention to provide a simple and economical two state output- .amplifying circuit which is extremely sensitive to small input signals.

In accordance with one embodiment of the invention,

-a bistable amplifier includes first and second parallel connected electric amplifying valves commonly connected to a constant current path, a third electric amplifying valve driven in response to the second valve, and a positive feedback circuit from the output of the third valve to the input of the second valve. In response to input signals applied to the first valve, the first and second valves are driven in opposite directions (one up, the other down) by reason of the constant current connection. The third valve drives a load to set or reset states depending upon which direction the firs-t valve is driven in by the input signals. The feedback circuit provides snap action and a latching function, and its parameters dictate the particular value of input signal required for reset.

Other and further objects of the present invention will become apparent from the following detailed specification taken in connection with the single figure drawing which illustrates a preferred embodiment of the invention.

In the amplifier shown generally at 10 in the drawing, there are four electric amplifying valves T1, T2, T3, and T4, each having a control electrode, a first type power electrode, a second type power electrode, and a power current path extending through the valve from the first type power electrode to the second type power electrode. The term first type power electrode is adopted as a generic term covering collectors and anodes and other equivalent electrodes in transistors, electronic tubes, and other electric valves. The term second type power electrode is adopted as a generic term covering emitters, cathodes and other equivalent electrodes in transistors, electronic tubes, and other electric valves.

Although other suitable valves such as electronic tubes may be employed, the valves are shown as transistors by way of example. The respective control and first type and second type power electrodes are related to transistors for example as follows. For a transistor connected in common emitter configuration or in common collector configuration, the base is the control electrode, the collector is the first type power electrode, the emitter is the second type power electrode, and the power path is the collector-emitter current path. Each of the transistors T has an emitter E, a collector C, and a base B. The reference letters T, E, C, and B for a particular valve have the same associative numerical sufiix. For example the reference characters T2, E2, C2 and B2 are ass0- ciated with the same valve.

Transistors T1 and T2 which are shown as NPN type by way of example, are connected in a differential amplifier arrangement 12, wherein two parallel connected circuits 14 and 16 are connected in series with a constantcurrent path 18 which includes a resistor 20 that puts a constant current constraint on the path 18 and the parallel arrangement of circuits 14 and 16. Resistor 20 has a relatively high impedance to provide to the path 18 a substantially constant current operation. Although passive in nature, the high impedance provides substantially the same effect as a constant current generator.

Circuit 14 includes the power path of transistor 1 and a collector resistor 22. Circuit 16 in the same manner includes the power path of transistor T2 in series with a collector resistor 23. The parallel arrangement of circuits 14 and 16 is connected across circuit points 24 and 25. Point 25 is connected to emitters E1 and E2 and also to the constant current path 18. Point 24 is conected to the positive terminal 26 of a power supply source 23 which includes batteries 30 and 32. The supply source 28 is provided with a negative terminal 34 and a common terminal 36 to which is connected a power supply common line 38. The constant current path 18 is connected between point 25 and the negative terminal 34.

Base B1 is connected to an input summing junction 4-) which is connected through reversely poled (oppositely related) diodes D1 and D2 to the power supply common 38. These diodes (D1 and D2) prevent the voltage at base B1 from exceeding a safe value relative to the baseemitter junction of transistor T1. An input terminal 42 for receiving a first volt-age V1 is connected through an isolating resistor 44 to the summing junction 40. In like manner a second input terminal 46 for receiving a second input signal V2 is connected through a resistor 48 to the summing junction 40. Common line 38 through an input terminal 50 is common to the input terminals 42 and 46. Thus input signal V1 is applied across terminals 42 and 50, while input signal V2 is applied across terminals 46 and 50. A capacitor 52 is connected between collector C1 and base B1.

Base B2 is connected through a resistor 54 to the power supply common line 38. A capacitor 56 is connected across the collector C2 and base B2. Capacitors 52 and 55 are optional, and may be employed to reduce the re. sponse speed of transistors T1 and T2 to make them less sensitive to spurious signals.

In the differential amplifier 12 the constant current of path 18 is divided between the paths 14 and 16 as a function of the difference between the voltages at bases B1 and B2. When bases B1 and B2 are at the same potential relative to common line 38, the constant current is divided equally between the branches 14 and 16. However if base B1 is more negative than base B2, transistor T1 will be driven toward cut 011, thus forcing a greater share of the constant current to flow through transistor T2. On the other hand, if base B1 is more positive than base B2, transistor T1 will be driven toward full conduction thus diverting a substantial portion of the constant current away from branch 16 and into branch 14 whereby branch 14 receives a greater share of the constant current than branch 16.

' From the above it is seen that in amplifier 12, the respective transistors T1 and T2 are concurrently driven in opposite directions (one upward, the other downward) in response to a net input signal to base B1 that creates Thus to concurrently drive transistor T1 upward (more a difference between the voltages on bases B1 and B2. conductive) and transistor T2 downward (less conductive) the input net signal must make base B1 more positive than base B2. Conversely, net input signal must drive base B1 more negative than base B2 in order to drive transistor T1 downward and transistor T2 upward. The push-pull drive in opposite directions may also be explained as follows. Due to emitter resistor 20 being common to transistors T1 and T2, transistor T1 operates as an emitter follower driving the emitter E2 of transistor T2. As a result of this action, if transistor T1 is driven upward, the increased conduction through transistor T1 causes point 25 to go more positive, thereby making emitter E2 more positive to drive transistor T2 toward cut-off (downward). The converse takes place when transistor T1 is driven downward (decrease conduction). Although transistor T2 responds to transistor T1, the action is so fast, that as a practical matter the inversely related drives of these transistors are considered concurrent and are so referred to herein.

Transistor T3, shown by way of example as a PNP type, is connected in a circuit configuration forming an amplifier 60 which is driven from the output circuit of transistor T2 by means of a coupling from collector C2 to base B3 through a protective diode D3. This diode protects the base-emitter-junction of transistor T3 against excessive reverse bias.

A collector resistor 62 is connected from collector C3 to the negative terminal 34 of the power supply. Emitter E3 is connected through an emitter resistor 64 to the positive terminal 26 of the power supply, and through a resistor 66 to the negative terminal 34 of the power supply. Resistors 64 and 66 form a voltage divider for establishing at emitter E3 a voltage which allows transistor T3 to be in its mid-conduction range. The ratio of resistor 62 to resistors 64 and 66 in parallel determines the gain of amplifier 60.

The relationship between amplifier 60 and transistor T2 is such that as transistor T2 is driven more conductive, transistor T3 is driven more conductive, and conversely when transistor T2 is driven less conductive, that is to or toward cutoff, transistor T3 is also driven less conductive.

Amplifier 60 drives a following amplifier 68 formed by transistor T4 and associated circuitry. Transistor T4 is shown as an NPN type by way of example. The output circuit of transistor T3 is coupled to the input circuit of transistor T4 by means of a connection from collector C3 to base B4. Emitter E4 is connected through an emitter resistor 70 to the power supply common 36. The output of transistor T4 drives a load 72, for example the operating coil 72 of a relay 74 provided with normally open contacts and normally closed contacts 76 and 78 for controlling a circuit 80. Collector C4 is connected through the relay operating winding 72 to the positive terminal 26 of the power supply. A diode D4 is a free wheeling diode to provide a discharge path for the inductive load. A diode D connected between emitter E4 and base B4 forms a protective shunt to guard transistor T4 against excessive reverse bias of its base-emitter junction.

The relationship between amplifier stages 60 and 68 is such that transistor T4 is driven to a relatively high conduction mode, for example saturation, in response to transistor T3 being in a relatively high conduction mode. Conversely, transistor T4 is driven to a relatively low conduction mode, for example cutoff, in response to transistor T3 being in a relatively low conduction mode.

Relay 74 is energized or picked up in response to transistor T4 being in a relatively high conduction mode. On the other hand, relay 74 is deenergized or dropped out in response to transistor T4 being in a relatively low conduction mode.

When the load 72 is energized (relay 74 picked up), the apparatus is in one state of operation, which for convenience may be referred to as the set state. On the other hand, when the load 72 is unoperated (relay 74 dropped out), the apparatus 10 is in a second state of operation which may be referred to as the reset state.

To provide snap action on set and reset, and to render the amplifier 10 stable in the set state of operation, a latch circuit in the form of a positive feedback circuit 82 provides positive feedback from the output circuit of amplifier 68 to the input circuit of transistor T2. Feedback circuit 82 includes resistor 54 and a resistor 84 that is connected between base B2 and emitter E4. Resistors 54 and 84 are connected in a series circuit with the power path of transistor T4 and the load 72 to form a voltage divider when, transistor T4 is conducting. The

voltage divider has an intermediate tap 86 at the junction between transistors 54 and 84, which tap is connected to base B2 to provide positive feedback to transistor T2. The positive feedback provides snap action and operates to firmly latch the apparatus in the set state. The positive feedback circuit 82 also accelerates the reset operation. Thus, the positive feedback provides snap action in both directions i.e., for the set and reset operations.

The feedback circuit 82 not only latches the apparatus in its set or operated state, but also provides a predictable and adjustable hysteresis zone relative to values of input signals required for pickup and dropout as will be further explained hereinafter.

In one successful operating example, the components of the circuit of FIG. 1 were as follows:

Transistors T1, T2 and T4 Type 2N16l3. Transistor T3 Type 2N1 132. Resistors 2G, 22, 24 Each kilohms. Resistors 4 4 and 48 Each 21.5 kilohms. Resistor 54 1O kilohms. Resistor 62 33 kilohms. Resistor 64 1 kilohm,

Resistor 66 3.3 kilohm. Resistor 7tl 100 ohms. Resistor 84 383 kilohms. Capacitors 52 and 56 Each .001 mfd. Batteries 30 and 32 Each 24 volts.

When there is no difference between the voltages at base B1 and base B2, the constant current from the constant current source 18 divides equally between branches 14 and 16. In this circumstance transistors T1 and T2 are conducting equally and the circuit relations are such that the output of transistor T2 holds transistor T3 at a conduction level in its mid-range, but which output level is insufiicient to drive transistor T4 to a sufiiciently high conduction level to operate the load 72 (pick up the relay 74). However, when an input signal drives B1 more negative than B2, even by a very small amount, transistor T1 is driven less conductive and transistor T2 is driven sufficiently conductive, by the resulting constant current diversion, to drive transistor T3 to a sufiiciently high level to, in turn drive transistor T4 to a sufficiently high level to operate the load 72 and pick up the relay 74.

More specifically, suppose that when the load 72 is deenergized or unoperated (relay 74 dropped out), the voltage at B2 is zero, and further suppose that the voltage at B1 is zero when the input signals V1 and V2 applied to input terminals 42 and 46 are each zero or are each of the same magnitude but of opposite polarity (assuming resistors 44 and 48 are equal to each other). The constant current will divide equally between branches 14 and 16 to produce conduction levels in transistors T1 and T2 which may be termed quiescent levels because there is no difference between the voltages applied to B1 and B2, and consequently no net input signal to the apparatus. Under these conditions (no net input signal applied to base B1), the conduction levels of transistors T3 and T4 may also be considered quiescent conduction levels.

Assume that it requires a difference of at least 0.01 v-olt between bases B1 and B2 to drive transistors T1 and T2 concurrently in opposite directions to a sufficient degree to operate the amplifier 10 from one to the other of its respective states. Now suppose that input signal V1 is +100 volts, and input signal V2 is -100.1 volts, thereby dropping B1 to say 0.02 volt, B2 still being at zero volts, resulting in a difference of over 0.01 volt between bases B1 and B2. This drives transistor T1 to a relatively lower level of conduction, thus forcing a greater share of the constant current from the constant current source 18 to fiow through branch 16 and driving the transistor T2 to a higher level of conduction than its quiescent level. As a result base B3 is driven more negative, to drive transistor T3 above its quiescent level. This causes base B4 to go more positive, thereby driving transistor T4 to a conduction level above its quiescent, thus to energize the load 72 and pick up relay 74. When transistor T4 starts its move to a higher conduction level, the resulting positive feedback to base B2 makes the base voltage more positive, and cumulatively increases the drive to transistors T2, T3 and T4 to provide snap action in energizing the load 72 and picking up relay 74. The positive feedback locks transistor T2 at a relatively high level of conduction to latch the amplifier in the set state (relay 74 picked up).

If it is assumed that the feedback raises the base B2 to +0.02 volt, it will now require more than +0.02 volt (in the assumed example, at least +0.03 volt) on base B1 to raise this base sufiiciently more positive than base B2 in order to drive transistor T1 upward to a level of conduction higher than its quiescent level so as to drive transistor T2 down and thereby operate the amplifier 10 to its reset state (relay 74 dropped out). Base B1 may be driven to more than +0.02 volt, for example by making input signal V1 +100.12 volts, and input signal V2 l00.0 volts, thus making the voltage at base B1 over +0.03 volt.

With base B1 more positive than base B2, transistor T1 preempts more of the constant current from the constant current path 18, consequently reducing the share of constant current taken by transistor T2. Thus transistor T1 is driven to a higher conduction level than its quiescent, while transistor T2 is driven to a lower level of conductionthan its quiescent. (It should be recalled that quiescent relates to the condition when there is no difference between the voltages at bases B1 and B2, which in this example occurs when the net input signal to B1 is zero.)

As transistor T2 is driven down, base B3 becomes more positive, thus driving transistor T3 to a low conduction level. This drives base B4 more negative to drive transistor T4 toward cutoff thereby deenergizing the load 72 and dropping out relay 74. The apparatus 10 has now been reset. It should be noted that the set and reset states of amplifier 10 are stable states.

If it is desired to change the minimum requirement for the voltage on base B1 which will reset the apparatus, the value of the voltage supplied to base B2 by the positive feedback circuit =82 may be changed by altering the resistance values of either or both of resistors 54 and 84. The voltage V supplied to base B2 by the feedback network 82 may be determined by the following relation with base B2 under open circuit condition, that is with tap 8'6 disconnected from base B2.

where V is the voltage at tap 86 to be supplied by the net work to base B2,

V is the voltage across the series combination of resistors 54 and 84 (this is the voltage between emitter E4 and power supply common 36),

R is the resistance value of resistor 54, and

R is the resistance value of resistor 84.

For a given value of input voltages V1 and V2, the set or pickup point of the apparatus is a function of the ratio between the input resistors 44 and 48.

It should be understood that, while specific conductivity types of transistors are shown'by way of example, different conductivity types may be substituted using known substitution techniques.

The present invention provides an extremely simple, but highly sensitive electronic bistable amplifier which will switch from one to the other of set and reset states in response to net input voltages of the order of millivolts.

It is to be understood that the hereinbefore described arrangements are simply illustrative of the principles of the invention, and that other embodiments and applications, and changes in detail and overall form, are all within the spirit and scope of the invention.

I claim as my invention:

1. Apparatus for selectively providing either of two output states, said apparatus comprising first, second and third electric valves each having a power path, an input circuit and an output circuit, first and second parallel connected circuits, each including in series the power path of a different one of said first and second valves, a constant current path connected in series with said parallel arrangement, means including said constant current path for driving said first and second valves in inverse relation in response to input signals having at least certain minimum requirements applied to the control electrode of the first valve, circuit means coupling the output circuit of said second valve to the input circuit of the third valve whereby the third valve is operated in higher and lower conduction modes in response to said second valve being in a particular one and the other of higher and lower conduction modes, respectively, load means connected in the output circuit of the third valve and operable in first and second modes in response to said third valve being in its higher and lower conduction modes, respectively, and means for accelerating the drive of the third valve to its higher conduction mode and latching said apparatus in the output state wherein said load is operated in its said first mode, the last said means comprising a positive feedback circuit coupled between the output circuit of the third valve and the control electrode of the second valve.

2. The combination as in claim 1 wherein said constant current path comprises series impedance for providing a constant current constraint.

3. Apparatus for selectively providing either of two output states, said apparatus comprising first, second and third electric valves, each having a power path, an output circuit and a control electrode, first and second parallel connected circuits, each including in series the power path of a different one of said first and second valves, a constant current path connected in series with said parallel arrangement, each of said valves having respective higher and lower conduction modes of operation, said first valve being operable in one of its said modes in response to input signals having at least a first minimum requirement applied to its control electrode and in the other of its said modes in response to signals having at least a second minimum requirement applied to its control electrode, said second valve being operable in its higher and lower conduction modes in response to said first valve being in its lower and higher conduction modes, respectively, circuit means coupling the output circuit of said second valve to the control electrode of the third valve whereby the third valve is operated in one and the other of its said modes in response to said second valve being in one and the other of its said modes, respectively, said second valve being operable in its said one and said other modes in response to said first valve being in its said one and said other modes, respectively, load means operable in respective first and second modes in response to said third valve being in its said one and said other modes, respectively, and latch means for latching said apparatus in the output state wherein said load is operated in its said first mode, said latch means comprising a positive feedback circuit coupled between the output circuit of the third valve and the control electrode of the second valve, the value of the feedback provided by said feedback circuit dictating the particular value of said second minimum requirement of input signal required to operate the first valve in its said other mode.

4. The combination as in claim 3 wherein said first and second valves are transistors each having an emitter connected to said constant current path.

5. The combination as in claim 3 wherein said feedback circuit comprises a voltage divider including impedance means in series with the power path of said third valve, and means coupling the control electrode of said second valve to an intermediate point on said voltage divider.

6. The combination as in claim 3 wherein said constant current path comprises series impedance for providing a constant current constraint.

7. Apparatus for selectively providing either of two output states, said apparatus comprising first, second and third electric valves, each having a power path, an output circuit and a control electrode, first and second parallel connected circuits, each including in series the power path of a diiferent one of said first and second valves, a constant current path connected in series with said parallel arrangement, each of said valves having respective more and less conductive modes of operation, said first valve being operable in its more conductive mode in response to input signals having at least a first minimum requirement applied to its control electrode and in its less conductive mode in response to signals having at least a second minimum requirement applied to its control electrode, said second valve being operable in its more and less conductive modes in response to said first valve being in its less and more conductive modes, respectively, circuit means coupling the output circuit of said second valve to the control electrode of the third valve whereby the third valve is operated in its more and less conductive modes in response to said second valve being in its more and less conductive modes, respectively, load means connected in the output circuit of the third valve and operable in first and second modes in response to said third valve being in its more and less conductive modes, re-

spectively, and latch means for latching said apparatus in that one of its output states wherein said load is operated in its said first mode, said latch means comprising a positive feedback circuit coupled between the output circuit of the third valve and the control electrode of the second valve, the value of the feedback provided by said feedback circuit dictating the particular value of said first minimum requirement of input signal necessary to operate the first valve in its more conductive mode.

8. The combination as in claim 7 wherein said constant current path comprises series impedance for providing a constant current constraint.

References Cited by the Examiner OTHER REFERENCES Slaughter, The Emitter-Coupled Differential Amplifier;

IRE Transaction-Circuit Theory; March 1956; pages 51-53.

ARTHUR GAUSS, Primary Examiner.

I. C. EDELL. P. H. EPSTEIN. Assistant Examiners. 

1. APPARATUS FOR SELECTIVELY PROVIDING EITHER OF TWO OUTPUT STATES, SAID APPARATUS COMPRISING FIRST, SECOND AND THIRD ELECTRIC VALVES EACH HAVING A POWER PATH, AN INPUT CIRCUIT AND AN OUTPUT CIRCUIT FIRST AND SECOND PARALLEL CONNECTED CIRCUITS, EACH INCLUDING IN SERIES THE POWER PATH OF A DIFFERENT ONE OF SAID FIRST AND SECOND VALVES, A CONSTANT CURRENT PATH CONNECTED IN SERIES WITH SAID PARALLEL ARRANGEMENT, MEANS INCLUDING SAID CONSTANT CURRENT PATH FOR DRIVING SAID FIRST AND SECOND VALVES IN INVERSE RELATION IN RESPONSE TO INPUT SIGNALS HAVING AT LEAST CERTAIN MINUMUM REQUIREMENTS APPLIED TO THE CONTROL ELECTRODE OF THE FIRST VALVE, CIRCUIT MEANS COUPLING THE OUTPUT CIRCUIT OF SAID SECOND VALVE TO THE INPUT CIRCUIT OF THE THIRD VALVE WHEREBY THE THIRD VALVE IS OPERATED IN HIGHER AND LOWER CONDUCTION MODES IN RESPONSE TO SAID SECOND VALVE BEING IN A PAR- 